Thin-film photovoltaic devices typically comprise a thin-film absorber layer structure sandwiched between front and back electrode layer structures. In the course of manufacturing such a device, pattering process steps are carried out, typically in order to achieve a separation of a large module area into individual cells while providing for an integrated electric series connection between these cells. Often, a structure of first patterning lines is referred to as P1 patterning and corresponds to a division of the back electrode layer into neighbouring parts. A structure of second patterning lines, P2 patterning, corresponds to an electrical connection between the front and back electrode layer structures, and the third patterning lines, P3, correspond to a separation into neighbouring parts of at least the front electrode layer.
A particular class of thin-film photovoltaic devices has an absorber layer formed of a group I-III-VI semiconductor, also referred to as a chalkopyrite semiconductor. Such a semiconductor is generally of the copper indium diselenide (“CIS”) type, wherein this expression is to be understood such that indium can be partly or fully replaced by gallium and/or aluminium, and selenium can be partly or fully replaced by sulphur. CIS type semiconductors include those characterized by the formula CuInxGayAl(1-x-y)SezS(2-z), wherein x+y≦1 and z≦2. Special cases of a CIS type layer are e.g. also denoted as CIGS or CIGSS. The CIS type layer can further comprise a low concentration, trace, or a doping concentration of one or more further elements or compounds, in particular alkali such as sodium, potassium, rubidium, caesium, and/or francium, or alkali compounds. The concentration of such further constituents is typically 5 wt % or less, preferably 3 wt % or less.
The CIS type layer is typically arranged on a back electrode of a metal film such as Mo. In a particular configuration the back electrode layer is supported by a substrate, typically soda-lime glass, but other substrates are possible as well. Preferably a diffusion barrier layer such as of silicon nitride or silicon oxide is arranged between the glass substrate and the back electrode.
The thin-film photovoltaic device is typically completed by depositing a buffer layer of e.g. cadmium sulphide on the CIS type layer, followed by one or more layers of a transparent metal oxide, often zinc oxide on which metal contacts are arranged. The top layer of the metal oxide is arranged to be an n-type semiconductor, e.g. by doping ZnO with boron or aluminium. The electrodes are provided with contacts or bus bars, and the cell is typically encapsulated and provided with a transparent front cover such as of glass.
An example of a manufacturing process of a CIS-type photovoltaic device is discussed in the paper “Rapid CIS-processing for high-efficiency PV-modules: development towards large area processing”, V. Probst et al., Thin Solid Films vol. 387 (2001) p. 262-267, incorporated herein by reference in its entirety. In the known method, the P2 patterning is done by mechanical scribing.
In mechanical scribing, a scribing tool such as a sharpened and mechanically hardened stylus tip is run over a surface, in the case of a P2 patterning step to cut trenches through the absorber layer structure. Although mechanical scribing has proven to be a suitable technology, practical experience has shown that the final contact resistance between front and back electrode layers can vary significantly along the patterning lines.
Laser patterning, similar to what is already in use for the P1 patterning step, has been tried for P2 patterning as well, but did not provide reliable results. Laser patterning requires precise knowledge of the (local) physical parameters of the layer to be patterned, such as absorption depth, heat dissipation, and this turns out to be problematic with high reliability. When a relatively thick layer of the order of more than one micrometer is removed, the risk of cutting not deep enough, or in fact too deep into the underlying structure, is therefore significant.